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OFDM consideration in the RF Front end of receivers!!

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imar

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ofdm frontend

hi freinds!

what are creteria that should be taken into account when designing an RF FRONT END knowing that the modulation scheme used is for an OFDM transmission?
i didn't really find a convincing response for this question ?

need your great help to make it clear;

many thanks

regards

imar
 

ofdm front end

Imar

the requirements for the RF front end are always the same: bandwidth, gain, noise, linearity.

Particular attention has to be taken in gain flatness and in linearity, as the ofdm signal can have an high peak-to-average ratio.

Mazz
 

cofdm rf front end

Mazz said:
the ofdm signal can have an high peak-to-average ratio.

Mazz

hi Mazz and thank you to be the first to reply!
As you said, one of the major problerms with OFDM is its PAPR :peak to average power ratio, i found that there are ONLY solutions in the baseband after conversion A/D using diffrent algorithms to alleviate this problem.
But, in analog part, to reduice the impact of this parameter, did the receiver need to have only a flat gain and a better linearity factor to take in account this PAPR?

thanks again.

regards

imar
 

ofdm snr requirement

The RF System analysis (and requirements) will give you the performances that the front-end shall meet.
For example you cannot define the front-end Noise Figure without considering the receiver sensitivity and SNR requirements for a specific modulation.
Generally OFDM use different types of modulations, e.g. SNR is BPSK=6dB, QPSK=11dB, 16QAM=16dB, 64QAM=24dB
Receiver NF[dB] = 174+ Receiver Sens[dBm] – 10*LOG(BW[Hz]) – SNR[dB]
 

rf frontend consideration

vfone said:
Generally OFDM use different types of modulations, e.g. SNR is BPSK=6dB, QPSK=11dB, 16QAM=16dB, 64QAM=24dB
Receiver NF[dB] = 174+ Receiver Sens[dBm] – 10*LOG(BW[Hz]) – SNR[dB]

thanks vfone.

this is the kind of statement that i'm trying to know and understand.
So a direct impact of an OFDM signal could be reflected on the noise figure.

if you go now more deeper, i think that even when designing RF blocs there are other requirements that maybe shoulb taken in account to maintain the orthogonality of subcarriers especially: linearity of the LNA and mixer.the offset frequency of the Loacal oscillator...
is it true?
if yes, are there other requirements that i did not mentionned above.

thanks for help

imar
 

ofdm rf front end

Referring only to the receiver front-end block (T/R switch, Filters, LNA, first mixer, first LO) parameters as noise figure, linearity (IIP3, IIP2), input dynamic range, blocking (inband and outband), phase noise, frequency stability, must take in consideration.
 

Great! That is the point that i am trying to be familiarized with.
And as you mentioned before, concerning the relationship that correlates OFDM with noise figure: these requirements that you stated above are very interestening.
so is there any references that emphasize the relationship between OFDM and other parameters that you stated to make these things more clear and to see with clarity what are the precise points for each requirement that shoould take in account an OFDM scheme.

sorry and thanks for help again.

imar
 

Until you get to the A/D Converter, a Receiver for OFDM and a receiver for a wideband modulation are identical. You have to worry about gain flatness across the signal bandwidth, but that's the case for any wideband signal. Noise figure, linearity are all derived in a similar fashion as for any receiver.

The LO phase noise for OFDM is typically specified with separate requirements for phase noise within the OFDM carrier spacing, and overall. There are some OFDM specific phase noise performance impairments that are related to phase noise for individual tones, vs. phase noise on all tones. I'm not very familiar with the details on that, though.

Dave Bengtson
 

imar

sorry for delayed answer, although I've seen that the discussion has proceed correctly.

It seems that your concern is on possible degradation of OFDM signal. I do agree with Dave, there is no "special" feature on OFDM respect to a general wideband modulation.

In the example you were discussing (NF-> OFDM parameter), you will not find any special reference.

If you need an RF subsystem study, this is a different issue we can discuss.

Mazz
 

Mazz said:
It seems that your concern is on possible degradation of OFDM signal.
Mazz

Hi Mazz!
you haven't to be sorry, really thank you to re-establish the discussion with you again. it is becoming more and more useful!
you made the stress on the main point i wanted to inderstand: in what part of the receiver ( Front end blocks) the OFDM signal could be affected ?
if we understand clearly these points, so the Rf requirements could be then explained as it was explained by vfone.

Mazz said:
If you need an RF subsystem study, this is a different issue we can discuss.
Mazz

it could be really helpful to understand more issues about these aspects.

thanks for help

regards

imar
 

Imar

generally speaking, not only relating to OFDM, the correct design flow should start from system or standard specifications to derive the architecture and the sub-specs of the receiver and/or the transmitter.

It is not a simple task, as it imlies at least 3 different skills: system, RF/analog & DSP.
If you want to limit it to the RF/analog, at least you should know, for the receiver, the required SNR at ADC.

Do you have all these info for your specific system?

OK, if you want to stay general, for an OFDM receiver, most challenging specs are the one related to Local oscillator (s) used in downconversion. If this is too high, the subcarriers of OFDM will be affected.

furthermore, being the OFDM an high peak-to-average signal, you should target linearity of your analog circuits taking into account also the peak of this signal.

Gain flatness is also a strong spec, but there are commonly used baseband algorithms that can compensate it.

What else? Many other things, but to go in detail, all the mentioned study has to be done.

A suggestion: if you are thinking to a full integrated solution, a very good book to study has been written by qizheng gu (google it), it's the only book that explain very well it for CDMA (but you can apply it to any modulation).

Mazz
 

Thanx Mazz!!

your explanations are so clear. You 've targeted points that i wanted to know.
Especially when spoke about standard's requirements.
So as you mentioned, now it could be possible to undrestand why there are some requirements that are crutial and severe as it presents the way to cope with the type of the signal that could be processed in the front end before reaching the ADC.

thanx for the book reference , i downloaded the one named RF_System_Design_of_Transceivers_for_Wireess_Communications written by qizheng gu.

Best regards

imar

:D
 

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